1. Field of the Invention
This invention relates to a magnetic recording and reproducing apparatus and more particularly to an apparatus using a magnetic head for both recording and reproducing with the head switched over between recording and reproducing operations by a switching circuit.
2. Description of the Prior Art
Until now, the apparatus of the above-stated kind has been arranged as follows: FIG. 1 of the accompanying drawings shows, by way of example, the arrangement of the essential parts of the conventional apparatus of this kind. In this example, the apparatus gives a still picture by recording with a fixed magnetic head, a video signal on a rotating magnetic sheet while forming circular recording tracks on the magnetic sheet with one field portion of the video signal recorded in each of these tracks. Then one field portion of the video signal is reproduced continuously with the magnetic head. The apparatus is so arranged that it begins to reproduce the track immediately after recording completion, permitting confirmation of the recorded picture by means of a monitor or the like. FIGS. 2(a) and 2(d) show, in a timing chart, the wave forms of the various parts "a" to "d" shown in FIG. 1. The operations of these parts of the apparatus will be understood from the following description: Referring to FIG. 1, the apparatus includes an input terminal 1 which receives a video signal to be recorded; a recording amplifier 2; a magnetic head 3; a capacitor 4; a magnetic sheet 5; a motor 6; an operation switch 7; a control signal generating circuit 8 which generates a control signal for switching-over between recording and reproducing operations; transistors 9 and 10 which function as analog switches; a reproduction pre-amplifier; and an output terminal for producing a reproduced video signal.
In recording, the control signal generating circuit 8 supplies the transistor 9 with a low level control signal and the transistor 10 with a high level control signal. The transistor 9 thus turns off and the other transistor 10 turns on. The video signal received via the terminal 1 is amplified up to a predetermined level by the recording amplifier 2. A DC component of the amplified video signal is removed by the capacitor 4. The signal is then passed through the magnetic head 3 as a recording current. As a result, a current flows to the transistor 10. Then, a magnetic field generated at the magnetic head 3 is recorded in a circular recording track on the magnetic sheet 5. Meanwhile, the motor 6 rotates the magnetic sheet 5 at a predetermined rotation frequency.
In reproduction, a high level control signal is supplied to the transistor 9 and a low level control signal to the transistor 10. The transistor 9 turns on and the transistor 10 turns off. A signal reproduced by the magnetic head 3 from the magnetic sheet 5 is amplified to a suitable level by the reproduction pre-amplifier 11 and is produced from the terminal 12 as a reproduced video signal. The magnetic sheet 5 in this case is, of rotated by the motor 6 at the same rotational frequency.
Referring now to the timing chart of FIGS. 2(a) to 2(d), the switch-over from a recording operation to a reproducing operation is as follows: At the time of recording, the transistor 9 is off and the transistor 10 on. Therefore, the potential of a part "c" of FIG. 1 is equal to the DC component of the voltage output of the recording amplifier 2 (let us assume that this potential is Vc). The potential of a part "d" of FIG. 1 is at about a ground potential GND. Upon completion of recording, a signal which instructs reproduction is supplied from the operation switch 7 to the control signal generating circuit 8 at a point of time t1 shown in FIGS. 2(a) to 2(d). Then, the level of the control signal supplied to the transistor 9 becomes high and that of the control signal supplied to the transistor 10 becomes low. Therefore, the transistor 9 turns on and the transistor 10 turns off. At this time, the potential on one side "c" of the capacitor 4 drops to the ground potential GND while keeping the potential difference between the two sides of the capacitor 4 at a value Vc. As a result, the potential on the other side "d" of the capacitor 4 drops to a minus potential value. At this time, the low level control signal is supplied to the base of the transistor 10. However, since the collector potential of the transistor 10 becomes lower than the emitter potential, the transistor 10 operates in the reverse direction. In other words, the electric charge accumulated at the capacitor 4 is discharged by a loop formed by the transistor 9, the capacitor 4, the head 3 and the transistor 10. Accordingly, a spike-like current flows to the head 3. Therefore, after recording completion, the head 3 further performs recording based on this spike-like current. As a result, a part of the signal recorded is erased. When the recorded signal is reproduced, the still picture thus obtained has a portion of the picture missing.
FIG. 3 shows the essential parts of an apparatus of this kind as another example of the prior art arrangement. In this case, a current due to an electric charge accumulated at a capacitor 4' is arranged so that it is not allowed to flow to the head 3, by removing the capacitor 4' from a part between the transistors 9 and 10 which function as analog switches. In accordance with this arrangement, the above-stated spike-like current does not flow at the time of switch-over from recording to reproduction. However, if the amplitude of a recording signal is large at the time of recording, the transistor 9 might operate in a direction reversed from its operating direction, since the potential of part "e" shown in FIG. 3 is close to the ground potential GND. In that event, the recording signal would be clipped.
It is an object of the present invention to solve the above-stated problems of conventional magnetic recording and reproducing apparatus in a simple and effective manner.
It is another object of the invention to provide a magnetic recording and reproducing apparatus in which no distortion takes place in a recording current and, after completion of a recording, no spike-like current is produced.
These and further objects and features of the invention will become apparent from the following detailed description of the embodiments thereof taken in conjunction with the accompanying drawings.